from itertools import permutations

n = int(input())
prob = [list(map(int, input().split())) for _ in range(n)]

cnt = 0
for p in permutations([i for i in range(n)]):
    x = prob[p[0]][0]
    for i in range(1, n):
        if x >= prob[p[i]][1]:
            break
        x = max(x + 1, prob[p[i]][0])
    else:
        cnt += 1

print(cnt)